$\begingroup$These videos are related and might be interesting, they are explorations of proposals of a similar scale and type, but they aren't quite close enough to warrant being part of an answer: youtube.com/watch?v=TEdsQmjLMKs & youtube.com/watch?v=i70wkxmumAw It's worth noting that diverting the gulf stream is a disastrous outcome and possibly is one that would occur in this case too.$\endgroup$
– Dubber RuckyFeb 18 '17 at 20:19

$\begingroup$@DubberRucky Good videos, but the Gulf Stream doesn't go through the English Channel. It heads north up the coast of Ireland and south down the coast of Spain and Africa.$\endgroup$
– SchwernFeb 18 '17 at 20:48

2

$\begingroup$Can you narrow this down? What kind of effects are you concerned about.$\endgroup$
– James♦Feb 18 '17 at 22:04

$\begingroup$It was a geological eyeblink ago (Last ice age) when sea level was lower and that land bridge existed. So from a global long term perspective, not much change.$\endgroup$
– nigel222Feb 19 '17 at 18:14

1 Answer
1

Let's see what it would take.

At its narrowest point, the Strait of Dover, that's 33 km across so lets build there. Its depth varies from 20 to 70m, so let's take a nice average of 40m. What would it take to do that on land, let alone water? Could we build even a 33 km, 40m high wall without currents and water to wash it away?

We can't make a traditional arch dam which gets its strength from spanning the gap with an arch. 33km is far too wide. We can't use a gravity dam, we're not holding back an artificial reservoir against gravity. It'll have to be an embankment dam, basically a long pile of rocks.

The first problem is when you pile up rocks into a wall they don't like to stay that way. They turn into a big pile. The angle this pile takes is called the angle of repose.

How much material do we need for a 33km long, 40 meter high wall, piled at a 45° angle? The area of a triangle is half the base times the height. We know the height, 40m, what's the base?

/|\
/ | \
/ 4| \
/ 0| \
/ m| \
/45° | 45° \
--------------
40m + 40m

45° is a right triangle, so its base will equal the height. We have two stacked together, so the base is 80m. 1/2 * 80m * 40m is 1600m2.

That's the area of the cross section of our dam. What's its volume? How many cubic meters do we need? 33,000 m * 1600m2 is 52.8 million m3 or 5.28e7 m3. That puts it in league with the largest dams in the world. A very big project, but totally doable.

Despite being so long, its relatively short for a dam. Since the thickness of the base, and thus amount of material needed, rises as the dam gets higher, a taller dam needs exponentially more material than a shorter one. Double the height and you quadruple the material needed.

That would be on land. There's tremendous engineering challenges to building a dam under water, but the currents in the English channel are fairly mild. I am not a civil engineer, but approximating it as a pile rocks seems reasonable.

What would its effect be?

Well, it would piss off anyone who owned a boat that needs to go through the English Channel. They'd have to go all the way around the North Sea, a quite longer and more treacherous journey. Presumably there would be a seaway built in.

Ocean currents through the Channel flow into the North Sea, they would be all but cut off. It's not like the North Sea would be cut off from the World Ocean, so it would be relatively minor. You'd need an oceanographer to explain exactly, what the effects would be, probably some change in salinity and migration of sea creatures. I doubt it would be good.

Politically and militarily it would be like the channel tunnel but more so. You now have a solid land bridge between the UK and the mainland allowing increased immigration and trade made even more difficult by the UK's current attitude towards the EU. There would be the question of who owns and administers the bridge, the UK or France?

Militarily, an island nation is no longer an island. In an emergency they can collapse the Channel Tunnel, but the dam is a very stable pile of dirt. Any damage to it would be superficial and repairable. While it would serve as a very nasty choke point to an invading army, its still easier than trying to invade by air and sea.

Speaking of the sea, it would restrict the movement of navies through the channel. They'd have to use a narrow seaway which would become a focus in a military conflict.

$\begingroup$This whole answer is wrong. There's no dam. The Channel is at sea level and water pressure is equal on both sides.$\endgroup$
– SRMFeb 19 '17 at 2:18

1

$\begingroup$Calling it a Dam instead of a Landbridge doesn't change the validity of the answer.$\endgroup$
– Miguel BartelsmanFeb 19 '17 at 2:29

$\begingroup$@Srm not exactly. There would be tidal phase differences between the tide travelling down the North Sea and the tide in the English channel. There would also be different storm surges. So there would be some water pressure to resist.$\endgroup$
– nigel222Feb 19 '17 at 18:19

$\begingroup$With current political dynamics this is a non-starter. Being an island nation is very deep in our psyche. The EU never understood this, hence brexit....$\endgroup$
– nigel222Feb 19 '17 at 18:25

1

$\begingroup$@SRM You are technically correct! The best kind of correct. It is a land bridge, but you'd build a land bridge the same way (with much hand-waving of details and protesting about not being a civil engineer). I drew on dams to illustrate that you couldn't build it like an arch or gravity dam so it would take a lot more material than you'd think. I thought the answer would end there with an unreasonable amount of material, but it turned out reasonable. It seems a good first order approximation.$\endgroup$
– SchwernFeb 19 '17 at 18:59